In printing machines having a ductor roller, the ductor roller is provided between a fountain roller and an ink transfer roller. The ductor roller is a roller divided into multiple individual rollers along its axis, and the duty ratio at which the individual rollers contact on the fountain roller is controllable independently for the individual rollers. Further, the printed density on the printed product is measured for each color component, and the individual rollers in the ductor roller are feedback controlled so that the printed density is in agreement with the desired density. With the feedback control, the variations in printed density during printing is reduced (Patent Document 1: JP 2015-63071A, corresponding to U.S. Pat. No. 9,446,581).
The ductor roller has an amount of ink drawn from the fountain roller, and ink transfer rollers also have an amount of ink. Because of the ink reservation on the rollers, the control of printed density by means of the ductor roller has a delay time. To reduce the delay time, it has been proposed to increase temporarily the amount of ink feeding to the ductor roller when the printing plate is exchanged and the new image area ratio in the new printing plate is increased, and also to decrease temporarily ink feeding amount to the ductor roller when the new image area ratio is decreased (Patent Document 1: JP 2015-63071A, corresponding to U.S. Pat. No. 9,446,581).